facebook - Mystery of "this" in ReactJS

Question

I am fairly new to the Facebook's React world. Their documentation seems to be very good but there are a few areas where I need a little bit of clarity. This is one of them.

Src: http://tuts-javascript.appspot.com/reactjs-add-remove-table-row

    var CompanyApp = React.createClass({
    getInitialState: function() {
      return {companylist:this.props.companies};
    },
    handleNewRowSubmit: function( newcompany ) {
      this.setState( {companylist: this.state.companylist.concat([newcompany])} );
    },
    handleCompanyRemove: function( company ) {
      
      var index = -1;   
      var clength = this.state.companylist.length;
        for( var i = 0; i < clength; i++ ) {
            if( this.state.companylist[i].cname === company.cname ) {
                index = i;
                break;
            }
        }
        this.state.companylist.splice( index, 1 );  
        this.setState( {companylist: this.state.companylist} );
    },
    render: function() {
      var tableStyle = {width: '100%'};
      var leftTdStyle = {width: '50%',padding:'20px',verticalAlign: 'top'};
      var rightTdStyle = {width: '50%',padding:'20px',verticalAlign: 'top'};
      return ( 
        <table style={tableStyle}>
          <tr>
            <td style={leftTdStyle}>
              <CompanyList clist={this.state.companylist}  onCompanyRemove={this.handleCompanyRemove}/>
            </td>
            <td style={rightTdStyle}>
              <NewRow onRowSubmit={this.handleNewRowSubmit}/>
            </td>
          </tr>
      </table>
      );
    }
  });
  
  var CompanyList = React.createClass({
    handleCompanyRemove: function(company){
      this.props.onCompanyRemove( company );
    },
    render: function() {
      var companies = [];
      var that = this; // TODO: Needs to find out why that = this made it work; Was getting error that onCompanyDelete is not undefined
      this.props.clist.forEach(function(company) {
        companies.push(<Company company={company} onCompanyDelete={that.handleCompanyRemove} /> );
      });
      return ( 
        <div>
          <h3>List of Companies</h3>
          <table className="table table-striped">
            <thead><tr><th>Company Name</th><th>Employees</th><th>Head Office</th><th>Action</th></tr></thead>
            <tbody>{companies}</tbody>
          </table>
        </div>
        );
    }
  });
  
  var Company = React.createClass({
    handleRemoveCompany: function() {
      this.props.onCompanyDelete( this.props.company );
      return false;
    },
    render: function() {
      return (
        <tr>
          <td>{this.props.company.cname}</td>
          <td>{this.props.company.ecount}</td>
          <td>{this.props.company.hoffice}</td>
          <td><input type="button"  className="btn btn-primary" value="Remove" onClick={this.handleRemoveCompany}/></td>
        </tr>
        );
    }
  });
  
  var NewRow = React.createClass({
    handleSubmit: function() {
      var cname = this.refs.cname.getDOMNode().value;
      var ecount = this.refs.ecount.getDOMNode().value;
      var hoffice = this.refs.hoffice.getDOMNode().value;
      var newrow = {cname: cname, ecount: ecount, hoffice: hoffice };
      this.props.onRowSubmit( newrow );
      
      this.refs.cname.getDOMNode().value = '';
      this.refs.ecount.getDOMNode().value = '';
      this.refs.hoffice.getDOMNode().value = '';
      return false;
    },
    render: function() {
      var inputStyle = {padding:'12px'}
      return ( 
        <div className="well">
          <h3>Add A Company</h3>
        <form onSubmit={this.handleSubmit}>
          <div className="input-group input-group-lg" style={inputStyle}>
            <input type="text"  className="form-control col-md-8"  placeholder="Company Name" ref="cname"/>
          </div>
          <div className="input-group input-group-lg" style={inputStyle}>
            <input type="text"  className="form-control col-md-8" placeholder="Employee Count" ref="ecount"/>
          </div>
          <div className="input-group input-group-lg" style={inputStyle}>
            <input type="text"  className="form-control col-md-8" placeholder="Headoffice" ref="hoffice"/>
          </div>
          <div className="input-group input-group-lg" style={inputStyle}>
            <input type="submit"  className="btn btn-primary" value="Add Company"/>
          </div>
        </form>
        </div>
        );
    }
  });
  var defCompanies = [{cname:"Infosys Technologies",ecount:150000,hoffice:"Bangalore"},{cname:"TCS",ecount:140000,hoffice:"Mumbai"}];
  React.renderComponent( <CompanyApp companies={defCompanies}/>, document.getElementById( "companyApp" ) );

This is a very good basic explanation of how ReactJS works. Thanks to the author.

But this comment,

var that = this; // TODO: Needs to find out why that = this made it work; Was getting error that onCompanyDelete is not undefined

Why is that necessary? Is this the right way to do it? If not, what is?

Thanks in advance.

Answer 1

There's no mystery of "this" that is specific to ReactJS.

This is just a case of standard scoping issues that crop up with callbacks in JavaScript.

When you're in a react component, all methods on the base component will be scoped with the this being the current component, just like any other JavaScript "class".

In your snippet you have a render method which is a function on the base component and therefore this is equal to the component itself. However within that render method you're calling a callback with this.props.clist.forEach, any function callbacks inside the render method will need to be either bound to the correct this scope, or you can do var that = this (although this is an anti-pattern and should be discouraged)`.

Example, slightly simplified version of your snippet:

var MyComponent = React.createClass({
    handleCompanyRemove: function(e) {
        // ...
    },
    render: function() {
        // this === MyComponent within this scope

        this.props.someArray.forEach(function(item) {
            // this !== MyComponent, therefore this.handleCompanyRemove cannot
            // be called!
        })
    }
})

As you can see from the comments above, inside your callback for the .forEach you cannot use this directly without either defining a variable outside, or properly binding the function.

Other options to solve this are:

Binding the callback function to the correct this scope. Example:

this.props.someArray.forEach(function(item) {
    // this === MyComponent within this scope too now!
    // so you can call this.handleCompanyRemove with no problem
 }.bind(this))

If you're using Babel/ES6 you can use the Fat Arrow function syntax which guarantees that this scope continues to the callback from the parent scope. Example:

this.props.someArray.forEach((item) => {
    // this === MyComponent within this scope too now!
    // so you can call this.handleCompanyRemove with no problem
})